1. Field of Invention
The present invention relates to cold rolled steel sheet suitably used for automobiles, household electrical appliances, and machinery, and particularly to a high tensile cold-rolled steel sheet having an ultrafine grain structure and exhibiting excellent characteristics including strength, ductility, toughness, strength-ductility balance, and stretch flangeability.
2. Description of Related Art
Steel sheets used for automobiles, household electrical appliances, and machinery are required to have excellent mechanical properties, such as strength, formability, and toughness. In order to enhance these mechanical characteristics comprehensively, it is effective to make the grain of the steel fine. Accordingly, many methods have been proposed for achieving an ultrafine grain structure.
As for high tensile steel sheets, it has recently been desired to manufacture a high functional steel sheet at a low cost. In particular, steel sheets for-automotive application are desired to have impact resistance as well as high strength, from the viewpoint of the protection of occupants in a crash.
Moreover, automotive steel sheets are required to have excellent press formability because many of them are press-formed into automotive parts. In addition, members and reinforcements for enhancing the strength of automobile bodies are often formed through the use of stretch flange formation. Accordingly, steel sheets for these automotive applications are highly desired to have excellent stretch flangeability as well as high strength.
According to these circumstances, grain fining of a high tensile steel is a challenge with the goal of preventing degradation of ductility, toughness, durability, and stretch flangeability, which are degraded as tensile strength becomes higher.
Large-reducing rolling, controlled rolling, controlled cooling, and the like have been known as methods for grain fining. As for large-reducing rolling, some methods for grain fining are disclosed in which austenite grains are subjected to large deformation to promote xcex3-xcex1 strain induced transformation, in Japanese Unexamined Patent Application Publication No. 53-123823 and Japanese Examined Patent Application Publication No. 5-65564, and others.
A precipitation strengthened steel sheet containing Nb or Ti is an example of application of controlled rolling and controlled cooling. This type of steel sheet is produced by making use of precipitation strengthening effect of Nb or Ti to increase the strength of the steel and, further, by making use of recrystallization suppressing effect of Nb or Ti so that xcex3-xcex1 strain induced transformation of non-crystallized deformed austenite grains reduces the grain size of ferrite crystal grains.
In addition, a method for producing a structure mainly containing isotropic ferrite has been disclosed in Japanese Unexamined Patent Application Publication No.2-301540. According to this method, part or the whole of a steel material partially containing ferrite is inversely transformed to austenite having an ultrafine grain size by heating the steel material to a temperature of the transformation point (Ac1 point) or more while being subjected to plastic deformation, or by heating the steel material and subsequently allowing it to stand at a temperature of Ac1 point or more for a predetermined period of time. Then, the resulting fine austenite grains are transformed to ferrite during subsequent cooling, thus resulting in a structure mainly containing isotropic ferrite grains having an average grain size of 5 xcexcm or less.
All of the techniques described above are intended for use in a hot-rolling process, that is, intended to reduce the grain size of a hot rolled steel sheet.
However, very few techniques for cold-rolled steel sheets are known, which have a thickness smaller than that of hot-rolled steel sheets and are required to have highly precise thickness and surface properties or subjected to galvanization or tinning, and in which the grain size is reduced in a conventional cold-rolling and annealing process.
A dual phase steel sheet having a combined structure of ferrite and martensite is typically known as a high-strength steel sheet with excellent formability.
Also, a highly ductile steel sheet utilizing transformation induced plasticity resulting from retained austenite is going into practical use.
These steel sheets hardened by hard second phase have high elongationability. However, the steel structure has a large difference between the hardnesses of ferrite, acting as the matrix thereof, and hard martensite (retained austenite also transforms into martensite in the deformation), acting as a major strengthening factor therein. This large hardness difference can cause voids and reduce the local elongation, thus deteriorating the stretch flangeability.
Accordingly, an object of the present invention is to provide a cold-rolled steel sheet having an ultrafine grain structure which is used for automobiles, household electrical appliances, and machinery, and a method for advantageously manufacturing the same. The cold-rolled steel sheet of the present invention is enhanced in the strength, ductility, toughness, strength-ductility balance and stretch flangeability by reducing the grain size thereof.
The inventors of the present invention have carried out intensive research to accomplish the object, and consequently, have obtained an ultrafine grain structure having an average grain size of 3.5 xcexcm or less by controlling the recrystallization temperature and A1 and A3 transformation temperatures of a steel sheet whose metal contents have been appropriately controlled, and then by controlling the recrystallization annealing temperature after cold-rolling and the cooling rate after the recrystallization annealing. Also, the inventors have found that the stretch flangeability of the resulting steel sheet can be extremely enhanced by optimizing the secondary phase of the steel structure.
Accordingly, the present invention is directed to a cold-rolled steel sheet having an ultrafine grain structure including a ferrite phase. The cold-rolled steel sheet includes: 0.03 to 0.16 mass percent of C; 2.0 mass percent or less of Si; at least one of 3.0 mass percent or less of Mn and 3.0 mass percent or less of Ni; at least one of 0.2 mass percent or less of Ti and 0.2 mass percent or less of Nb; 0.01 to 0.1 mass percent of Al; 0.1 mass percent or less of P; 0.02 mass percent or less of S; 0.005 mass percent or less of N; and Fe and incidental impurities. The ferrite phase has a content of 65 percent by volume or more and an average grain size of 3.5 xcexcm or less. The C, Si, Mn, Ni, Ti, and Nb satisfy expressions (1), (2), and (3):
637.5+4930(Ti*+(48/93)xc2x7[%Nb]) greater than A1xe2x80x83xe2x80x83(1)
A3 less than 860xe2x80x83xe2x80x83(2)
[%Mn]+[%Ni] greater than 1.3xe2x80x83xe2x80x83(3)
where
Ti*=[%Ti]xe2x88x92(48/32)xc2x7[%S]xe2x88x92(48/14)xc2x7[%N]xe2x80x83xe2x80x83(4)
A1=727+14[%Si]xe2x88x9228.4[%Mn]xe2x88x9221.6[%Ni]xe2x80x83xe2x80x83(5)
A3=920+612.8[%C]2xe2x88x92507.7[%C]+9.8[%Si]3xe2x88x929.5[%Si]2+68.5[%Si]+2[%Mn]2xe2x88x9238[%Mn]+2.8[%Ni]2xe2x88x9238.6[%Ni]+102[%Ti]+51.7[%Nb]xe2x80x83xe2x80x83(6)
[%M] represents element M content. (mass %)
Preferably, a remainder content of the steel sheet, other than the ferrite phase, is limited to 3 percent by volume or less except for bainite.
Preferably, the cold-rolled steel sheet further includes at least one of 1.0 mass percent or less of Mo and 1.0 mass percent or less of Cr.
Preferably, the cold-rolled steel sheet further includes at least one element selected from the group consisting of Ca, rare earth elements, and B in a total amount of 0.005 mass percent or less.
The present invention is also directed to a method for manufacturing a cold-rolled steel sheet having an ultrafine grain structure. The method includes: reheating a starting steel material to a temperature of 1200xc2x0 C. or more; hot-rolling the starting steel material; cold-rolling the hot-rolled material; performing recrystallization annealing at a temperature in the range of A3xc2x0 C. to (A3+30)xc2x0 C.; and cooling the annealed material to 600xc2x0 C. or less at a rate of 5xc2x0 C./s or more. The starting steel material includes: 0.03 to 0.16 mass percent of C; 2.0 mass percent or less of Si; at least one of 3.0 mass percent or less of Mn and 3.0 mass percent or less of Ni; at least one of 0.2 mass percent or less of Ti and 0.2 mass percent or less of Nb; 0.01 to 0.1 mass percent of Al; 0.1 mass percent or less of P; 0.02 mass percent or less of S; 0.005 mass percent or less of N; and Fe and incidental impurities. The C, Si, Mn, Ni, Ti, and Nb satisfy expressions (1), (2), and (3):
637.5+4930(Ti*+(48/93)xc2x7[%Nb]) greater than A1xe2x80x83xe2x80x83(1)
A3 less than 860xe2x80x83xe2x80x83(2)
[%Mn]+[%Ni] greater than 1.3xe2x80x83xe2x80x83(3)
where
Ti*=[%Ti]xe2x88x92(48/32)xc2x7[%S]xe2x88x92(48/14)xc2x7[%N]xe2x80x83xe2x80x83(4)
A1=727+14[%Si]xe2x88x9228.4[%Mn]xe2x88x9221.6[%Ni]xe2x80x83xe2x80x83(5)
A3=920+612.8[%C]2507.7[%C]+9.8[%Si]3xe2x88x929.5[%Si]2+68.5[%Si]+2[%Mn]2xe2x88x9238[%Mn]+2.8[%Ni]2xe2x88x9238.6[%Ni]+102[%Ti]+51.7[%Nb]xe2x80x83xe2x80x83(6)
[%M] represents element M content. (mass %)
Preferably, the method includes further cooling the cooled material from 500 to 350xc2x0 C. for a period of time in the range of 30 to 400 s, after cooling the material to 600xc2x0 C. or less at a rate of 5xc2x0 C./s or more.
Preferably, the starting steel material further includes at least one of 1.0 mass percent or less of Mo and 1.0 mass percent or less of Cr.
Preferably, the starting steel material further includes at least one element selected from the group consisting of Ca, rare earth elements, and B in a total amount of 0.005 mass percent or less.
According to the present invention, a high tensile steel sheet having an ultrafine grain structure and exhibiting excellent mechanical properties, and particularly strength-elongation balance, toughness, and stretch flangeability, can advantageously manufactured stably without extensively modifying equipment.